Hydric Species Research Articles

Woody species response to altered herbivore pressure at Isle Royale National Park

AbstractHerbivores shape vegetative communities via numerous mechanisms, including browse. We used vegetation monitoring data from Isle Royale National Park to examine woody species change across a nine‐year interval, coinciding with herbivore escalation. Here, moose and snowshoe hare are the dominant herbivores, while the gray wolf is the apex predator. Our initial sampling period (2010) followed six years of low moose abundance, while our second sampling event (2019) followed a nine‐year escalation in moose density. We tested for change in both saplings and shrubs and compared diameter size distributions of common tree species in three island sections. We found a decline in large saplings of sugar maple, a species limited to the west section of the island. We also saw declines in small saplings of sugar maple, paper birch, and trembling aspen. For some species, including black spruce and white spruce, taxa that are unpalatable to moose, diameter distributions were proportionally larger (i.e., indicative of fewer small individuals) in 2019 than in 2010. In contrast, diameter distributions of black ash in the central section and trembling aspen in the central and east sections of the island were proportionally smaller (indicative of more small individuals) in 2019 than in 2010. We found that ~50% of common shrub taxa were more abundant during the second sampling event, while none declined. Our work here brings to light several unanticipated results. While we demonstrated partial recovery of trembling aspen, likely originating during the earlier period of low moose abundance, our results suggest further changes may be attributable to other island herbivores. The response of black ash, a hydric species, may be stemming from a sixfold increase in active beaver sites, while that of spruce may be attributable to a historic spike in snowshoe hare density. We currently have only a limited understanding of the impacts of the island's lesser studied herbivores on overstory regeneration. Further research on these linkages can inform decisions addressing controls on forest structure.

What does not kill you can make you stronger: Variation in plasticity in response to early temporally heterogeneous hydrological experience

AbstractTemporally heterogeneous environments may drive rapid and continuous plastic responses, leading to highly variable plasticity in traits. However, direct experimental evidence for such meta‐plasticity due to environmental heterogeneity is rare.Our objective was to investigate the effects of early experience with temporally heterogeneous water availability on the subsequent plasticity of plant species in response to water conditions.We subjected eight plant species from three habitats, four exotic and four native to North America, to initial exposure to either a first round of alternating drought and inundation treatment (Ehet, temporally heterogeneous experience) or a consistently moderate water supply (Ehom, homogeneous experience), and to a second round of drought, moderate watering or inundation treatments. Afterwards the performance in a series of traits of these species, after the first and second rounds of treatments, was measured.Compared withEhom,Ehetincreased final mean total mass of all species considered together but did not affect mean mortality.Ehetrelative toEhom, decreased the initial total mass of native species as a group, but increased the mass of exotic species or species from hydric habitats;Ehetalso increased the late growth of natives, but did not for exotics, and increased the late growth of mesic species more than xeric and hydric species.Our results suggest that previous exposure to temporal heterogeneity in water supply may be not beneficial immediately, but can be beneficial for plant growth and response to water stress later in a plant's lifetime. Heterogeneous experiences may not necessarily enhance the degree of plasticity but may improve the expression of plasticity in terms of better performance later, effects of which differ for different groups of species, suggesting species‐specific strategies for dealing with fluctuating abiotic environments.Synthesis. Previous temporally heterogeneous experience can benefits plant growth later in life though modulating the expression of plasticity, leading to adaptive meta‐plasticity. Studies of meta‐plasticity may improve our understanding not only on the importance of variable plasticity in relation to how plants cope with environmental challenges but also on the costs versus benefits of plastic responses and its limits over the long term.

Resilience to extreme flooding shown by both hydric and mesic wetland plant species

AbstractGlobal climate change models forecast an increasing frequency and duration of extreme flood events, including during the growing season. In this mesocosm experiment, the survival, growth, and flowering of two hydric and two mesic wetland plant species were monitored under two extreme flood regimes, namely, repeated 2‐ and 7‐day floods, and compared with unflooded conditions. Plant survival was not significantly affected by flooding, but species showed different growth and flowering responses to the flood regimes. The hydric species Cardamine pratensis showed contrasting responses to floods with significantly more flowering stems and longer leaves in the 2‐day regime but delayed and poorer flowering in the 7‐day regime. Juncus articulatus, the other hydric species, responded most actively to 7‐day flooding, with significantly longer leaves, taller and more abundant flowering stems, and more flowers than in unflooded conditions. The mesic species Ranunculus acris showed variable growth and phenological responses to flooding, whereas Scorzoneroides autumnalis was most affected by the 7‐day flood regime, producing significantly shorter leaves and flowering stems and fewer flowers earlier in the season, compared with unflooded conditions. Overall, repeated 7‐day floods had a greater impact on plant performance than 2‐day flood events. All four species showed resilience to extreme flooding, irrespective of whether they were classed as hydric or mesic, but there was differential tolerance between species. This suggests that wetlands should be able to sustain vegetation under flooding extremes induced by climate change but community composition, biodiversity, and wetland services will all be affected.

Effects of climate-induced increases in summer drought on riparian plant species: a meta-analysis.

1. Frequency and duration of summer droughts are predicted to increase in the near future in many parts of the world, with considerable anticipated effects on riparian plant community composition and species richness. Riparian plant communities along lowland streams are characterised by high species richness due to their system-specific environmental gradients. As these streams and their hydrological gradients are mainly rain-fed, they are sensitive to precipitation changes.2. We conducted a literature survey and meta-analysis to examine the effects of an increase in summer drought on: (i) riparian plant biomass; (ii) riparian seedling survival and (iii) riparian plant species composition and richness. We also aimed to determine whether hydrological thresholds related to drought tolerance can be distinguished for riparian plant species.3. ISI Web of Knowledge was searched for relevant peer-reviewed studies, and 23 papers were found that met our criteria and contained quantitative study results. To detect overall responses of biomass and seedling survival, a random-effects model was applied using Comprehensive Meta-analysis™ software. Regression curves were then fitted to response ratio data relating the effects on drought-impacted groups to those on control groups.4. Our results showed that a drought duration of approximately >30 days strongly reduces riparian plant biomass and that a duration of approximately >30–35 days and high drought intensities (starting from 3 to 4 cm water table decline per day) can be detrimental for riparian seedling survival. Especially Populus and Salix seedlings showed a reduced survival in response to drought, in contrast to Tamarix seedlings, which have the ability to rapidly and expansively elongate their roots. The data also revealed that an increase in drought conditions rapidly leads to a decline of riparian species richness and an increased presence of species adjusted to drier conditions.5. Riparian groundwater level, surface water permanence and certain plant traits, especially plasticity in rooting depth, were mentioned most frequently as factors determining species responses. Very few studies mentioned hydrological thresholds, such as critical values for ground- and/or surface water levels, and so far these results have proved difficult to generalise.6. Our meta-analysis has shown that the projected increase in the duration and intensity of drought periods, especially intense droughts lasting more than 30 days, can be expected to narrow the riparian wetland zone with typical hydric species and accelerate riparian wetland species losses in the near future. This may require extra efforts in terms of management and restoration of species-rich riparian areas.

Riparian vegetation of two semi-arid Mediterranean rivers: Basin-scale responses of woody and herbaceous plants to environmental gradients

Mediterranean fluvial ecosystems are subjected to ever-increasing water demands and to a wide variety of other human impacts with potential negative effects on riparian vegetation. Nevertheless, few studies have compared the importance of human versus natural factors in structuring riparian vegetation in this region, particularly in semi-arid areas. We examined basin-scale responses of riparian vegetation to the main environmental gradient extracted by canonical correspondence analysis in two semi-arid Mediterranean rivers in southern Spain, considering different community attributes (woody, herbaceous, functional groups, and exotics species) and using different metrics (species composition, richness, and cover). The results show two main environmental gradients for riparian vegetation: a prime altitudinal gradient of increasing salinity towards the lower sites, and a second gradient of increasing water pollution and human physical impact parallel to the hydroperiod shortening. Species richness of woody and herbaceous life forms, and cover of woody species, reacted negatively to the increasing salinity downstream, a pattern that was driven by most functional groups. This is consistent with the key role of salinity for vegetation. Furthermore, as for other semi-arid riparian ecosystems, the decline in rainfall coupled with higher temperatures and evaporative stress at the lower elevation open-canopy sites may have constrained herbaceous species. Woody richness and cover declined with shorter hydroperiod and increasing human impacts, a response that was driven by mesic and hydric species. These results agree with the idea that surface flow permanence is an outstanding limiting factor for woody vegetation in semi-arid rivers, possibly exerting more influence on species richness than flood disturbance. However, herbs showed very little response to drought or human impacts compared to woody species, which may be related to the higher recovery rate from disturbance of herbaceous life forms, due to their shorter life-span and higher colonization rates. Generally, exotic species were positively affected by dissolved nutrients (N and P), especially exotic herbaceous richness and cover by nitrogen. As in other Mediterranean rivers, percentage richness of exotics in our rivers was substantially lower compared to fluvial ecosystems in other regions.

Use of a Wetland Index to Evaluate Changes in Riparian Vegetation After Livestock Exclusion1

Abstract: A method was developed to characterize ecological integrity of riparian sites based on the abundance of hydric species. This wetland index can be calculated with species data, or with community type data as performed here. Classified riparian community types were used to describe vegetation at 14 livestock exclosures and adjacent grazed areas. Community type wetland index values were generated and used to calculate site wetland index values. It was hypothesized that removal of livestock would result in higher wetland index values because of release from herbivory and decreased physical disturbance of vegetation, streambanks, and soil. The wetland index for exclosures was about 12% higher than grazed sites; differences were statistically significant (p < 0.01) based on pairedt‐tests. The increase in hydric vegetation after livestock exclusion may have contributed to the greater bank stability (p = 0.002) and smaller width‐to‐depth ratio (p = 0.005) in exclosures. Challenges were encountered in using community types to describe and compare site vegetation, which could be avoided with species data collection. The wetland index can be a tool to monitor sites over time, compare sites with similar environments, or compare sites for which environmental differences can be accounted.

Rapid responses of vegetation to hydrological changes in Taylor Slough, Everglades National Park, Florida, USA

We analyzed the dynamics of freshwater marsh vegetation of Taylor Slough in eastern Everglades National Park for the 1979 to 2003 period, focusing on cover of individual plant species and on cover and composition of marsh communities in areas potentially influenced by a canal pump station (“S332”) and its successor station (“S332D”). Vegetation change analysis incorporated the hydrologic record at these sites for three intervals: pre-S332 (1961–1980), S332 (1980–1999), post-S332 (1999–2002). During S332 and post-S332 intervals, water level in Taylor Slough was affected by operations of S332 and S332D. To relate vegetation change to plot-level hydrological conditions in Taylor Slough, we developed a weighted averaging regression and calibration model (WA) using data from the marl prairies of Everglades National Park and Big Cypress National Preserve. We examined vegetation pattern along five transects. Transects 1–3 were established in 1979 south of the water delivery structures, and were influenced by their operations. Transects 4 and 5 were established in 1997, the latter west of these structures and possibly under their influence. Transect 4 was established in the northern drainage basin of Taylor Slough, beyond the likely zones of influence of S332 and S332D. The composition of all three southern transects changed similarly after 1979. Where muhly grass (Muhlenbergia capillaris var. filipes) was once dominant, sawgrass (Cladium jamaicense), replaced it, while where sawgrass initially predominated, hydric species such as spikerush (Eleocharis cellulosa Torr.) overtook it. Most of the changes in species dominance in Transects 1–3 occurred after 1992, were mostly in place by 1995–1996, and continued through 1999, indicating how rapidly vegetation in seasonal Everglades marshes can respond to hydrological modifications. During the post-S332 period, these long-term trends began reversing. In the two northern transects, total cover and dominance of both muhly grass and sawgrass increased from 1997 to 2003. Thus, during the 1990’s, vegetation composition south of S332 became more like that of long hydroperiod marshes, but afterward it partially returned to its 1979 condition, i.e., a community characteristic of less prolonged flooding. In contrast, the vegetation change along the two northern transects since 1997 showed little relationship to hydrologic status.

Effects of short-term dehydration and rehydration on photosynthesis and respiration by Antarctic bryophytes

The hypothesis that rates of carbon exchange and recovery following dehydration by Antarctic bryophytes are related to habitat water availability was investigated. Carbon fixation was measured using an infra-red gas analysis system. As the water content of the bryophytes was reduced, respiration rates fell less quickly than those for gross photosynthesis. As a result, net photosynthesis moved from positive to negative, before tending to zero. Xeric species maintained a greater percentage of their photosynthetic capacity at reduced water contents than hydric species, although this trend was not reflected in terms of absolute carbon fixation. Comparison of the experimental observations with measurements of field water contents suggested that water contents of hydric and mesic species remained above those required to maintain maximal rates of photosynthesis through most of the growing season, whereas photosynthesis by xeric species was often water-limited. Recovery following rehydration demonstrated the typical bryophyte resaturation respiration burst and slower recovery of photosynthesis. Times taken to reach the compensation point were generally longer than those reported for non-polar species. Recovery was faster in xeric than in hydric species, although there was no correlation with the final degree of recovery. The results partially support the hypothesis tested, and provide a basis for the inclusion of water content and desiccation events in models of Antarctic bryophyte productivity.

Effects of continuous and repeated dehydration on carbon fixation by bryophytes from the maritime Antarctic.

The effects of dehydration and rehydration on carbon exchange in 14 bryophytes from the maritime Antarctic were investigated using an infra-red gas analysis system. Continuous long-term (1-12 months) and repeated (1-6 one-month cycles) desiccation responses were investigated under controlled conditions. Loss of photosynthetic rate increased with length of dehydration period in all species, although some desiccation tolerance was observed even in those bryophytes from the most hydric habitats. Percentage retention of photosynthetic rate increased from hydric to xeric species, but this pattern was not repeated in terms of absolute rates of carbon fixation due to the high initial rates in the hydric species. Repeated cycles caused a greater loss of photosynthetic rate than continuous dehydration in hydric species, but the opposite situation occurred in mesic and xeric mosses. The latter groups were possibly better able to utilise the short periods of rehydration during cycles. In most bryophytes an increase in the percentage loss of photosynthetic rate following dehydration-rehydration occurred from spring to summer to autumn samples. This pattern was clearest in the hydric species and reduced in the xeric species. These variations were largely due to changes in the initial rates of photosynthesis during the growing season. It is suggested that this increased photosynthetic capacity is stress-sensitive, and is lost during either desiccation or winter freezing; the base photosynthetic capacity, being stress-tolerant, survives either of these events. The results obtained support the hypothesis that water availability is of importance in determining the distribution of bryophytes in the Antarctic. However, only the broad scale of variation in plant communities could be explained by these observations; other factors must be important in determining the finer scale of species distribution and community composition. The results are applicable to attempts to model the productivity of Antarctic bryophytes from known or predicted environmental data.

An experimental study of growth in relation to morphology and shoot water content in maritime Antarctic mosses.

The dependence of shoot growth and growth form on water availability was studied experimentally in six species of maritime Antarctic moss. Under all conditions the largest growth increments were observed in the hydric species Brachythecium austro-salebrosum and Drepanocladus uncinatus. The xeric Andreaea depressinervis grew the least. Lateral shoot production varied within and between species. Over 50 % of the biomass produced in D. uncinatus was derived from lateral shoot production, whereas Polytrichum alpestre produced very few lateral shoots and A. depressinervis produced none. Leaf density and leaf size also varied with total water content. In all species growth ceased at total water contents of 100% d. wt or less. However, the total water content at which maximum growth was observed differed between species. Racomitrium austro-georgicum (mesic/xeric) had the lowest optimum for growth at 370% of d. wt and D. uncinatus (hydric) exhibited maximum growth between 890 and 2300% d. wt. Optimum total water contents for growth were greater than those at full turgor and published optima for net assimilation. Growth and total water content of these Antarctic mosses were similar to those reported for temperate species.

Vegetation Change in a Topogenic Bog Following Beaver Flooding

MITCHELL, C. -C. AND W. A. NIERING (Department of Botany, Connecticut College, New London, CT 06320-4196). Vegetation change in a topogenic bog following beaver flooding. Bull. Torrey Bot. Club 120: 136-147. 1993.-Vegetation change was documented along three permanent transects after nearly three decades of beaver flooding. The anchored forested wetland community was killed and replaced by a minerotrophic fen vegetation, whereas minimal change occurred within the tall scrub-shrub, dwarf shrub and open meadow comnmunities. Along all transects, tree cover was reduced twoto five-fold, whereas low heath shrub cover, especially Chamaedaphne calyculata and Kalmia angustifolia, increased. Herbaceous cover, primarily sedges, increased, especially in the newly created fen. Within the floating mat communities the evergreen ericaceous shrubs and scattered stunted Picea mariana and Larix laricina persisted as did the herbaceous bog flora. Species richness showed little change from 1960-1988 although there was a floristic shift toward more hydric species. Flooding differentially affected the vegetation patterm favoring more minerotrophic species in some areas, but did not appear to threaten the typical bog flora. Historically, beaver and other environmental influences have probably modified the vegetation development over millenia, including prolonged quiescent periods interrupted by periodic disturbances as documented in this study. For most peatlands this dynamic model appears more realistic than any orderly successional dogma.

The major organic components and caloric contents of high arctic bryophytes

Thirty-five bryophyte species representing the major habitats of a high arctic tundra lowland (Devon Island, Canada, 75° N) have been analyzed for contents of nitrogen, carbon, ash, and ether- and alcohol-soluble fractions. Mean contents (%) for the green portion of these species are total nitrogen, 1.00 (1.08 ash-free); total carbon, 45.9 (48.7 ash-free); ether-soluble fraction, 2.8 (3.0 ash-free); alcohol-soluble fraction, 11.8 (12.5 ash-free); and ash, 5.7. In the hydric species group, the total nitrogen content and the alcohol-soluble fraction are significantly greater than in mesic or xeric groups. When the green and brown segments are compared, the nitrogen content and the alcohol-soluble fraction are greater in the green portion, while the ash content is higher in the brown portion. The ash-free caloric values of hydric species range from 4.57 to 4.97 kcal/g, while the mesic and xeric species have a range from 4.50 to 4.69 kcal/g. The relations between habitats and between the green and brown portions are discussed in relation to general metabolic activity and productivity.

Vegetation of Califonia Fan Palm Oases on the San Andreas Fault

The vegetation of 24 oases located in the western Colorado Desert was composed of 78 species from 34 families, with an average of 11 species per oasis. Species distributions within an oasis were determined by available water and could be divided into three contoured belts and hydric zone had 10% of the total species, primarily hydrophytes, and accounted for one—fifth of the total plant cover. The oasis—proper zone contained phreatophytes and halophytes, comprising 17% of the total species and producing two—fifths of the cover. The remaining cover was produced by 73% of the species, mainly xerophytes, present primarily in the desert—oasis ectone. Oases were located on hillside seeps or in canyon washes. Although most dominant species were common to both types, they differed in vegetational composition. Hydric species were more common in wash oases. Species requiring stable sites with sub—surface water were more abundant in seep oases. All oases showed individuality since Washingtonia filifera was the only ubiquitous species. Haplopappus acradenius and Prosopis juliflora occurred in 75% of the oases; 10% of the species were restricted to two or three oases; and 33% were confined to one oasis. Eleven rare species were encountered. Absence of palm reproduction was due to inadequate water, lack of stimulating rains, or alkali soils. Alkali soils appear detrimental to plant growth. Optimum conditions for new growth need occur only once a century to sustain oases. Maximum palm ages appear to be about 200 years. Floods alter wash geomorphology and vegetation, but have a rejuvenating effect on dense, stagnating stands of hydrophytes. Fire determines oasis composition and affects the physiognomy, productivity, reproduction, and maintenance of the fire—tolerant palms.